1. Field of the Invention
The present invention relates to a shaft configuration of an idler gear in a conveyance unit of a recording apparatus.
2. Description of the Related Art
Among recording apparatuses such as printers, there exists a serial recording apparatus in which a recording head is mounted on a carriage configured to move in a direction orthogonal to a sheet conveyance direction, with sheet conveyance and recording on a sheet by the recording head being performed alternately.
In such a recording apparatus, a conveyance roller is arranged on the upstream side and a discharge roller is arranged on the downstream side of the recording head with respect to the sheet conveyance direction. To secure the requisite accuracy in sheet conveyance, the conveyance roller is equipped with a code wheel for detecting the rotation amount thereof and the recording apparatus main body is equipped with an encoder serving as a detection unit thereof.
Further, there exist three conveyance modes: a first mode in which sheet conveyance is performed solely by the conveyance roller at the sheet leading edge portion; a second mode in which sheet conveyance is performed by both rollers at the sheet central portion; and a third mode in which sheet conveyance is performed solely by the discharge roller at the sheet trailing edge portion. Recording is performed in each mode.
Due to limitations in terms of cost and space, there are many apparatuses in which, instead of being driven separately, the conveyance roller and the discharge roller are connected together by gears and driven by a single drive source. In this case, depending on the distance between the two rollers, a plurality of idler gears are arranged between the gears respectively provided on them, thereby effecting drive connection.
In such a construction, however, even when sheet conveyance is performed solely by the discharge roller after the sheet has left the conveyance roller, the conveyance amount is controlled based on the information of the code wheel provided on the conveyance roller. Thus, when the attitude of the idler gears is unstable as in the case, for example, of movement on the peripheral surface of a rotation shaft, the transmission accuracy of the gears deteriorates, which leads to deterioration in conveyance accuracy.
In view of this, there has been proposed a construction in which the support shaft of each idler gear is not a round shaft as in the prior art but one of an oval cross-sectional configuration consisting of flat portions and arcuate portions, and the gear inner peripheral surface is supported by two connection portions between the flat portion and the arcuate portions, whereby the attitude of the gear is stabilized.
However, the shaft as discussed in Japanese Patent Application Laid-Open No. 2006-35613 is formed by flat portions and arcuate portions, which means the a large portion has to be cut off in order to form the flat portion. Thus, as compared with an ordinary shaft, the shaft is thinner, with its strength being rather lower. This reduction in strength leads to an increase in the deflection of the shaft, which causes the gear to be deviated from a predetermined position, resulting in deterioration in transmission accuracy.
In a case where, instead of mounting a metal shaft to a metal plate as in Japanese Patent Application Laid-Open No. 2006-35613, a shaft is integrally formed with a molded frame taking the cost into consideration, forming the shaft in an oval cross-sectional configuration as in Japanese Patent Application Laid-Open No. 2006-35613 results in a further increase in shaft deflection, resulting in further deterioration in transmission accuracy.
An attempt to arrange the two support portions away from each other (in a wide angle with each other), the portion that has to be cut off in order to form the flat portion further increases. Thus, when the strength of the shaft is taken into account, there are limitations in terms of the arrangement of the support portions.
Further, when chamfering or rounding is effected in the case where the boundaries between the flat portion and the arcuate portions serve as the support portions, the gear will be deviated from the predetermined position. If wear is taken into account, although the degree of roundness should be larger, that would further increase the deviation of the gear from the predetermined position, making it difficult to secure a large degree of roundness.